1. Field of the Invention
The present invention relates to an image forming apparatus by electrophotography, such as a copier and a printer, and a method for controlling the image forming apparatus, particularly to an image forming apparatus having a function of adjusting image quality to stabilize printing quality and a method for controlling the image forming apparatus.
2. Description of the Prior Art
A conventional image forming apparatus by electrophotography is mounted with a developing apparatus for forming a toner image. The developing apparatus is roughly classified into one in mono-component developing system using mono-component toner and one in dual-component developing system using dual-component developer including mono-magnetic toner and magnetic carrier.
Since the developing apparatus in mono-component developing system is suitable for miniaturization but not suitable for high-speed development, most of high-speed and long-life image forming apparatuses employ the developing apparatus in dual-component developing system.
In this type of a developing apparatus using dual-component developer, carrier itself in dual-component developer is not consumed and remains in the apparatus, thus the carrier is not reduced, but toner is consumed by development and is reduced. Hence, in order to stabilize image quality, toner is appropriately supplied to maintain a constant toner concentration of dual-component developer.
Moreover, in the image forming apparatus, printing quality is changed influenced by deterioration of a photoreceptor or developer, change in the environmental condition, and the like, thus various technologies of adjusting image quality (referred to also as a process control technology) are disclosed in order to prevent the change in printing quality. In recent years, even when the image forming apparatus has not been used for a long time, these adjustment modes are performed.
As a conventional technology, proposed is an image forming apparatus in which adjustment of image quality is performed when returned from a reposed mode. Returning from the reposed mode means that an image formation engine portion has stopped until then, and, for example, in the case of dual-component developing system, agitating and charging of developer have not been performed for a certain time and the environmental condition, such as a temperature and a humidity under which the machine is installed, has been changed (difference in temperature/humidity in the morning and in the daytime, for example). When the environmental condition has been changed, a charge amount of developer, an electrical potential of a photoreceptor, or the like is considered to be changed. Due to such a change, change in image density or change in color tones is generated in printing processing after returning from the reposed, thus an image quality adjustment mode is executed to secure desired printing quality.
In addition, in the actual image quality adjustment mode, adjustment of an image density in the high density side and adjustment of a halftone density are performed, and in order to obtain optimum printing quality, it takes about 30 to 60 seconds to adjust the image quality. Particularly when a document of low printing rate is continuously printed, toner is hardly supplied, thus a charge amount of toner increases. Along with this, an operation is performed in the apparatus side so that a development bias is increased and an effective development potential is secured (a high development potential is obtained) in order to obtain a fixed image density or more.
However, since the charge amount of toner decreases when the apparatus has been reposed, an abnormally dense (high) image density is shown when development is performed in that state, thus the countermeasure thereto has been studied.
Hence, as a conventional technology 1, for example, disclosed is an image forming apparatus comprising: detection means for detecting adhered toner quantity as a toner image which is formed on a photoreceptor and has an almost highest image density in order to control the highest image density; developing means that is provided with a developer carrier which is rotated so as to feed developer and that visualizes an electrostatic latent image on the photoreceptor by adhering toner thereto; and image density adjustment means for adjusting an image density based on detection information of the detection means to a predetermined image density, wherein the image density adjustment means executes first adjustment in which a reference value of a rotational frequency of the developer carrier is changed according to a history of developer and second adjustment in which a changing amount of a development bias, which is applied when the highest image density is controlled, from the reference value is changed in accordance with the history of developer in different timings based on the history of developer (see Patent Literature 1: Japanese Patent Application Laid-Open No. 2001-154434).
In addition, as another conventional technology 2, disclosed is an image forming apparatus that optimizes a density control factor affecting an image density, in which, as a processing mode for optimizing the density control factor, a plurality of processing modes having different number of steps are configured so as to be selectively executable, and in addition, whether or not a reference concerning status change in the apparatus is satisfied is determined at the time of returning from a sleep, and a processing mode is selectively executed from the plurality of processing modes in accordance with the determination result (see Patent Literature 2: Japanese Patent Application Laid-Open No. 2003-177638).
However, in the conventional technology 1, adjustment is performed during a print job, which causes a problem that job efficiency is degraded. In addition, also in the conventional technology 2, since selection and execution of an optimum processing mode are performed in order to obtain desired printing quality when returning from a sleep mode, the process of selecting the processing mode is performed every time of returning from the sleep mode, which causes a problem that job efficiency is degraded.